1. Field of the Invention
The present invention relates to a quantum well laser diode, particularly to a high-efficiency, low drive-current quantum well laser diode.
2. Description of the Prior Art
In recent years, optical communication technology and optical information processing are playing major roles in various fields. For example, digital optical communication using optical fibers has made possible large increases in data communication densities, and optical disks and laser printers have produced a considerable expansion of the range of optical information processing applications.
The progress of optical communications and optical information processing technology owes much to advances made in the laser diodes used as light sources. The small size and high efficiency that are features of laser diodes have brought these devices into widespread use, for example as light sources for compact disk systems, video disk systems, bar code readers and optical communication systems.
In a laser diode the lasing action is generated by the injection of electrons into a P-N junction active layer. Recent advances in semiconductor technology such as MBE (molecular beam epitaxy) and MOCVD (metal-organic chemical vapor deposition) that make it possible to grow epitaxial layers as thin as 1 nm or less, have led to the realization of laser diodes that use quantum well active layers less than 20 nm thick, with higher levels of efficiency and lower drive current requirements (see W. T. Tsang in "Semiconductors and Semimetals" vol. 24, pp 397, Academic Press, San Diego (1987)).
While MBE and MOCVD have made it possible to grow epitaxial layers with relatively good uniformity, when using a laser diode to excite a solid state laser or fiber amplifier, it is necessary to set the frequency of the excitation laser diode to match the absorption frequency range of each of the media involved, for which a minimum frequency precision of 1 nm is desirable. As a consequence a very high order of epitaxial precision is required in the laser diode fabrication process. This is no easy task, and in practice it is a matter of selecting diodes having the required matching frequency In the case of quantum well laser diodes, in particular, care is needed as decreasing the well width of the quantum well raises the conduction band and valence band quantum level energy, reducing the oscillation wavelength, so the smaller the well width the greater the degree of change in the oscillation frequency based on the well width (see pp 213 of Physics and Applications of Semiconductor Superlattices, Physical Society of Japan Annals, Baifukan (1984)).
Among such quantum well laser diodes, recently there has been much work on the development of 980 nm laser diodes for use as the excitation light source of Er doped fiber amplifiers. Most of these are strained quantum well lasers on a GaAs substrate, with an InGaAs quantum well and AlGaAs/GaAs cladding layer or optical waveguide layer (see for example P. Bour et al, SPIE vol. 1219, Laser-Diode Technology and Application II (1990), pp 43).
However, with a strained quantum well, a high In content will produce an increase in the mismatch between the lattice constant of the surrounding AlGaAs/GaAs layers and that of the InGaAs quantum well and, moreover, increasing the thickness of the quantum well produces an increase in the overall forces acting on the quantum well. A larger strain therefore gives rise to lattice relaxation as transfers take place, causing high-density lattice defects that produce a sharp deterioration in crystallinity (see G. C. Osbourn in "Semiconductors and Semimetals" vol. 24, pp 459 (1987)). Therefore, while the oscillation wavelength can be controlled by changing the In content and the quantum well width, the range of this control is limited. Particularly with InGaAs/GaAs/AlGaAs system quantum well laser diodes, in order to extend the 830 nm to 870 nm oscillation frequency of an unstrained GaAs quantum well, it is necessary to increase the In content by a considerable amount or use a wider quantum well.
Thus, the fact that lengthening the wavelength increases the amount of strain means there is a limit to the degree by which the wavelength can be increased. In particular, the strain acting on the active layer of semiconductor lasers during laser operation is a cause of accelerated device degradation, so too large a degree of strain is therefore undesirable.